pH and Aquaponics

Feb 25, 2021 | Farm

By: Alex (Farm Intern)

What is pH? pH is an abbreviation for “potential of hydrogen” which comes from p standing for “power” and the standard capital H being the element hydrogen. pH is a measure of how acidic or basic/alkaline a solution (could be your water or soil) is. The pH scale is from 0-14 with 7 being neutral 0-7 being acidic and 7-14 being basic. pH is important for plants because it affects nutrient availability, soil bacteria, toxic elements, and more.

In aquaponic systems, maintaining a pH between 5.5 and 7 is the optimal choice for most plants, with some exceptions thriving in more acidic or basic environments. Another factor that is important in growing plants is nitrogen and ammonia levels. These levels can be determined by the bacteria in your farm. The bacteria needed to break down organic matter tends to be the most active between the pH range of 5.5-7. Nitrogen is released by organic matter that is broken down by nitrite-oxidizing and ammonia-oxidizing bacteria in the soil or water. These bacteria regulate the amounts of toxic ammonia and nitrite. The nitrite-oxidizing bacteria breaks down nitrite into nitrate which can be used by plants but can be toxic to fish at high levels.

Maintaining a pH of 5.5-7 will also allow for the most nutrients to be available for your plants. Nitrogen, potassium, and sulfur are not as directly affected by pH as other nutrients can be, but they can be low under extreme conditions. Phosphorus is affected greatly by pH value. When your medium (soil or water) is alkaline or has a pH of 7.5 and above, phosphorus tends to bind with calcium and magnesium to form a less soluble compound that plants may not be able to use. When pH is very acidic phosphorus tends to bind with aluminum and iron to form another less soluble solution. Micronutrients tend to not be as affected by pH but most tend to have larger amounts available in slightly acidic soils, that is a pH of 6.5-6.8.

In aquaponics pH is extremely important in maintaining healthy levels of nutrients for plants and to keep fish stress levels down. Plants tend to prefer slightly acidic pH values as it allows for the most nutrients to be available to them. Fish and the bacteria that breakdown nutrients on the other hand prefer a slightly basic pH. To compromise in aquaponics the pH of a system is typically slightly acidic in the range of 6.5-6.8 to keep both fish and plants happy.
Fish, primarily tilapia are popular among aquaponics farmers because of their ability to live in slightly harsher environments. These fish prefer a pH of 6.8-9. This means they can thrive in the same conditions plants prefer but when pH dips below 6, these fish can become stressed and die.
Testing your soil or waters pH is important and simple to keep the pH at an optimal level. One method of pH testing in an aquaponics system is as easy as putting a probe into your system’s water.
If you don’t have access to a pH probe, no worries, you can buy testing strips for less than $10.
If you are growing crops in soil you could send a soil sample to a lab, you could buy a test kit, or you could try a DIY test to get a general idea of your soils pH.
Once you know your pH is acidic, neutral, or basic you may need to change it to cater towards the crops you want to grow.
In aquaponics to change the pH you need to add mixtures of compounds to your system to balance it out. If your pH is low and you need to make your system’s water more basic you can add equal parts calcium carbonate and potassium carbonate to your system. If you add too much of those to your system you can just add phosphoric acid which is safe and will actually give your plants more of the phosphorus nutrient. Both of these methods could require trial error.
Just like in aquaponics adjusting your soil’s pH could require some trial and error as there are no standardized measurements for adjusting any given sized garden. To change a soil’s pH there are chemicals that can be used or you can use sulphur or limestone. To make a soil more acidic you can add a small amount of sulfur then retest your soil to find out if it’s at an optimal level and repeat as necessary. If your soil is too acidic to start or you accidentally added too much sulfur there is a solution. Adding garden lime or dolomite to your soil will reduce its acidity. Adding some to your soil and retesting just like with the sulfur will eventually get you to the right pH range.